A fuel cell directly converts the chemical energy of fuel into electric energy at a high conversion efficiency. In particular, a polymer electrolyte fuel cell can produce high output at a low temperature in comparison with a phosphoric acid fuel cell and the like. Therefore, the polymer electrolyte fuel cell is expected to be a small portable power supply such as an automotive power supply.
The polymer electrolyte fuel cell includes a stack formed by stacking single cells, two charge collectors provided outside the stack, and the like, each of the single cells including an electrolyte membrane formed of a polymer ion-exchange membrane such as a fluororesin ion-exchange membrane having a sulfonic acid group, catalytic electrodes supporting a catalyst such as platinum and provided on either side of the electrolyte membrane, separators provided with grooves used as gas passages for supplying a fuel gas such as hydrogen or an oxidant gas such as oxygen or air to the electrodes, and the like.
As shown in FIG. 1, the single cell includes a pair of electrodes 3 and 4 (cathode 3 and anode 4) disposed on either side of a polymer electrolyte membrane 5 formed of a fluororesin ion-exchange membrane, separators 1 formed of a dense carbon material and dosposed with the electrodes 3 and 4 interposed in between, and sealing materials 6 provided on the end of the separators in parallel with gas grooves. The electrodes 3 and 4 are formed of a porous body formed of carbon short fibers supporting a catalyst such as platinum, a product obtained by binding carbon black supporting a catalyst using a resin, or the like.
A number of linear or grid-like grooves 2 are formed in the separator 1. The space formed between the groove 2 and the cathode 3 is used as a passage for an oxidant gas (oxygen-containing gas such as air), and the space formed between the groove 2 and the anode 4 is used as a passage for a fuel gas (e.g. hydrogen gas or mixed gas containing hydrogen as the main component). A current is caused to flow between the electrodes by utilizing the following electrochemical reactions which occur when the fuel gas and the oxidant gas contact the electrodes. A cell stack is assembled by stacking several tens to several hundreds of single cells.Anode: H2→2H++2e−Cathode: ½O2+2H++2e−→H2OTotal reaction: H2+½O2→H2O
Therefore, since it is necessary to completely separately supply the fuel gas and the oxidant gas to the electrodes, the separator must exhibit high gas impermeability. Moreover, since it is effective to reduce the internal resistance of the cell in order to increase power generation efficiency, the separator is required to have a reduced thickness and exhibit high conductivity.
In order to improve the cell performance, it is important to prevent an increase in contact resistance between the separator and the electrode and prevent a leakage of gas between or from the single cells by assembling the stack so that the single cells closely adhere and maintain an excellent contact state during power generation. Specifically, the separator material is required to exhibit high strength so that breakage or deficiency does not occur during assembly, and to exhibit sufficient strength at a cell operating temperature (about 80 to 120° C.). Moreover, the separator material is required to exhibit high moisture resistance so that a dimensional change due to moisture absorption in air does not occur.
A carbon material is generally used as the separator material for which the above properties are required. A carbon/cured resin molded product is suitably used as the separator material which is produced by binding a carbon powder such as graphite using a thermosetting resin as a binder and molding the resulting product.
For example, JP-A-2000-021421 discloses a polymer electrolyte fuel cell separator member and a method of producing the same, wherein the separator member is formed of a graphite-cured resin molded product which is a plate-shaped molded product containing 60 to 85 wt % of a graphite powder having a particle size distribution with an average particle diameter of 50 μm or less and a maximum particle diameter of 100 μm or less, and 15 to 40 wt % of a thermosetting resin, and has a resistivity in the plane direction of 300×10−4 ohm·cm or less, a ratio of the resistivity in the thickness direction to the resistivity in the plane direction of 7 or less, and a flexural strength of 300 kgf/cm2 or more.
JP-A-2000-243409 discloses a polymer electrolyte fuel cell separator member and a method of producing the same, wherein the separator member is formed of a carbon-cured resin molded product containing 40 to 90 wt % of a carbon powder and 10 to 60 wt % of a thermosetting resin and having a flexural strength at room temperature of 30 MPa or more and a flexural strength decrease rate from room temperature to 100° C. of 30% or less.
JP-A-2004-127646 proposes a method of producing a polymer electrolyte fuel cell separator member, the method including mixing a phenolic resin solution, of which the cured product exhibits a saturated water absorption of 3% or less, and a graphite powder so that the resin solid content is 10 to 25 wt % and the graphite powder is 75 to 90 wt %, kneading the mixture, drying and grinding the kneaded product, filling a mold with the ground particles, and thermocompression-molding the ground particles. This patent document describes that a separator which is warped to only a small extent and shows an increase in electric resistance due to water absorption to only a small extent can be produced by using a phenolic resin with a low water absorption.
As a resin molding material suitable for fuel cell separators and the like, JP-A-2001-261935 discloses an epoxy resin molding material containing an o-cresol novolak epoxy resin, a bisphenol A epoxy resin, and artificial graphite as the essential components, and JP-A-2002-083609 discloses a fuel cell separator composition containing a graphite powder, an epoxy resin binder, and a curing accelerator, wherein the graphite powder is mixed in an amount of 5 to 15 times the weight of the epoxy resin binder, the epoxy resin binder contains an epoxy resin and an epoxy resin curing agent, and the epoxy resin binder has a viscosity of 0.01 to 0.5 Pa·s at 150° C. and has a viscosity of 3 Pa·s or more or is solid at 25° C.